Electric motors typically consist of a stator surrounding a substantially cylindrical rotor fixed to a shaft. The stator may have concentrated windings, typically consisting of copper wires coiled tightly around individual iron teeth extending inward toward the rotor. This type of stator winding construction results in relatively short end turns on the windings, as compared with distributed windings. Only a small amount of length along the axis of the motor must then be devoted to winding end turns, and most of the length can include stator teeth and be directly useful for producing torque.
Electro-mechanical transmissions generally require the use of two electric motors to transmit power to the output shaft of the transmission. Electric motors can generate a substantial amount of heat during operation. Electric motors with concentrated motor windings often generate more heat than motors with distributed windings and can be more susceptible to overheating which can lead to less efficient performance. Cooling the electric motor can positively influence motor efficiency and continuous torque. The performance issues include motor drag, motor parasitic losses in the coolant pumping, motor winding resistance, spin losses, and power rating.
Air or oil is most commonly used to carry heat away from the electric motors. Electric motor cooling can strongly influence transmission packaging and mass in vehicles. A motor cooling system incorporated into an electromechanical transmission should not substantially increase the axial or radial space required for the electric motor as space available for vehicle transmissions can be very limited.